The present invention relates to a circuit for broadband amplification. German Patent Document DE 37 09 169 A1 discloses an antenna coupler in which a first 90° hybrid coupler is connected to the input of the antenna coupler and a second 90° hybrid coupler is connected to the output of the antenna coupler. Two stop filters are connected in parallel between the first and the second hybrid coupler, each stop filter being connected in series to a phase shifter element. In this antenna coupler, the bandwidth of the signal to be transmitted is a function of the bandwidth of the particular 90° hybrid coupler. Broadband amplification is not possible using the known antenna coupler.
Linear broadband amplifiers (class A) are predominantly used in measuring technology as measuring amplifiers. In this case, multiple transistor stages adapted to broadband are connected one behind another to form an overall amplifier. The particular transistors receive a suitable standby current to ensure the operating point for linear amplifier operation. The predominantly used semiconductor technology for these amplifiers is GaAs. These measuring amplifiers typically operate very linearly, but have a restricted frequency range and are limited in saturation power.
To improve the broadband adaptation of the individual amplifier stages, HF negative feedback (RC element) is used between the HF input and output. This HF negative feedback dampens a growing oscillation tendency of the HF transistors, caused by their increased amplification characteristic (FIG. 1). The HF negative feedback also ensures uniform amplification behavior over a very wide frequency range. A loss of amplification is accepted.
Broadband 90° hybrid couplers are used to improve the input adaptation (FIG. 2). In addition, HF negative feedback is typically used for each amplifier stage. The input adaptation is thus improved over the frequency range of the 90° hybrid couplers. In addition, the saturation power of the amplifier is increased.
A circuit configuration for achieving power adaptation in noise-adapted high-frequency amplifiers is disclosed in German Patent Document DE 28 07 813 B1.
The mode of operation of a 3 dB 90° hybrid coupler is known. A 90° hybrid coupler acts, as indicated by the name, like a directional coupler which distributes half of the supplied HF power to each of two HF outputs. The HF signals at these two HF outputs have a phase difference of 90°. If HF power is reflected back into the 3 dB outputs, addition of the HF power only occurs at the insulated output. At this point, the reflected HF power may be converted into heat using a terminating resistor. Therefore, no reflected HF power may come back to the input of the 90° hybrid coupler, which results in a significant improvement of the input standing wave ratio VSWR of the overall circuit.
Broadband 90° hybrid couplers are very large in their dimensions. Anaren, Inc. provides a 90° hybrid coupler which is compatible with HF power up to 100 W cw over a frequency range of 500 MHz to 3.0 GHz. However, dimensions of approximately 15×3.5×1 cm are too large for development of small and compact transmitters. Although almost no reduction in size of such a coupler may be made from the physical aspect, the desire still exists of reducing the size. Furthermore, broadband hybrid couplers are limited to lower power levels in their maximum permissible power.
Kuhne Electronic provides broadband amplifiers. However, these amplifiers have an amplification having a bandwidth of less than one octave.
In the following, the term bandwidth is understood as the size of the frequency range which a signal occupies between its lower and upper limiting frequencies. Furthermore, the term octave is understood as an interval having a frequency ratio of 2:1 between an upper frequency value and a lower frequency value.
Exemplary embodiments of the present invention provide a compact circuit for broadband amplification, using which a broadband amplification of more than 1 octave is possible.
The circuit of the present invention includes an input HF_IN and an output HF_OUT. A first 90° hybrid coupler connected to the input HF_IN and a second 90° hybrid coupler connected to the output HF_OUT. Two amplifier stages are connected in parallel between the first and the second 90° hybrid couplers. The 90° hybrid couplers have an operating range of 1/n octaves between a lower frequency value and an upper frequency value, nεN\{0}, where n is an element of the natural positive whole numbers in such a way that the upper frequency value is greater than the lower frequency range and simultaneously less than two times the value of the lower frequency range, and the amplifier stages are transistors having a III-nitride, or SiC, or diamond basis.
In the circuit according to the present invention, 90° hybrid couplers are used, which have an operating range of 1/n octaves between a lower frequency value and an upper frequency value, nεN\{0}, i.e., n is an element of the natural positive whole numbers. In other words, the upper frequency value of the operating range is greater than the lower frequency value and simultaneously less than or equal to two times the value of the lower frequency range.
A 90° hybrid coupler having such an operating range is also referred to in the following as a narrowband hybrid coupler. In contrast to a broadband hybrid coupler, which has an operating range having an upper frequency value which is greater than two times the value of the lower frequency range of the operating range.
The present invention allows 90° hybrid couplers to be used both in SMD or drop-in construction and also as aluminum-housed 90° hybrid couplers. Drop-in components are understood by those skilled in the art as a directly fitting replacement part, i.e., replacement of an existing component, circuit, module, etc. within an existing circuit, by inserting a more modern or higher performance embodiment, without the infrastructure of the circuit board, the software, or anything else having to be modified for this purpose. An SMD component (surface mounted device) is understood by those skilled in the art as a surface-mountable component.
A possible SMD 90° hybrid coupler is, for example, the component Anaren Xinger 11306-3 and is offered by Anaren, Inc. These narrowband SMD 90° hybrid couplers are available for the mobile wireless market as a mass-produced product. Depending on how large the bandwidth of the coupler is selected, the power compatibility of the component also increases.
According to exemplary embodiments of the present invention, the amplifier stages are based on transistors which are manufactured from semiconductor materials having large electronic bandgaps. Such transistors are also referred to as wide bandgap transistors. Such transistors may be manufactured, for example, on the basis of III-nitride semiconductors, e.g., GaN, Aln, or on the basis of silicon carbide (SiC) or diamond.
Wide bandgap transistors are distinguished by a higher input impedance in contrast to typical transistors, e.g., based on GaAs (III-V semiconductors). In contrast to transistors based on GaAs, wide bandgap transistors do not tend to have vibration response in the range from 100 MHz-500 MHz. Wide bandgap transistors may thus also be used in this frequency range, while in contrast transistors based on GaAs tend to oscillate in this frequency range.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.